Described below is a refrigeration apparatus having at least                a warm connection element, which is thermally connected to parts of a device which are to be cooled,        a cold connection element, which is thermally connected to a heat sink,        a heat pipe made of a material with low thermal conductivity, which is connected at a first end to the warm connection element and at a second end to the cold connection element and whose interior is filled at least partially with a refrigerant which can be circulated by a thermosiphon effect.        
A refrigeration apparatus having the aforementioned features is disclosed, for example, by DE 102 11 568 B4.
Refrigeration systems, for example refrigeration systems for superconducting magnets, often utilize so-called bath cooling. A liquid refrigerant, for example helium, with a temperature of typically 4.2 K may be used for such bath cooling. However, large amounts of the corresponding refrigerant are required for bath cooling. In the case of a superconducting magnet, there is also the possibility that it will lose its superconducting properties, for example by exceeding a critical current or a critical magnetic field for the corresponding superconductive material. In such a case, a large amount of heat is developed in a short time by the superconductive material. In the case of bath cooling, the heat given off causes the refrigerant inside the cryostat to boil. Any gaseous refrigerant given off in large amounts leads to a rapid increase of the pressure inside the cryostat.
In order to counter this problem and at the same time reduce the costs for the refrigerant, cooling systems without a refrigerant bath have been designed. Such cooling systems can make do without any refrigerant. The refrigerating power is in this case introduced into the regions to be cooled merely by solid-state thermal conduction. In such a cooling system, the regions to be cooled may be connected to a refrigeration machine by a so-called solid-state cryobus, for example made of copper. Another option involves connecting the regions to be cooled and the refrigeration machine to a closed pipeline system, in which a small amount of refrigerant circulates. The advantage of such cooling systems without a refrigerant bath is furthermore that they are easier to adapt to mobile loads to be cooled, than cooling systems which have a refrigerant bath are. Cooling systems without a refrigerant bath are therefore suitable in particular for superconducting magnets of a so-called gantry, such as is used in ion radiation therapy for combating cancer. In the cooling system described above, the refrigerating power may be provided by a refrigeration machine having a cold head, in particular a Stirling refrigerator.
A superconducting magnet, in which a cold head is directly connected mechanically and thermally by its second stage to the holding structure of a superconducting magnet winding, is disclosed for example by U.S. Pat. No. 5,396,206. In the case of the aforementioned superconducting magnet, the required refrigerating power is introduced directly into the superconducting magnet windings by solid-state thermal conduction. If however it is necessary to replace a cold head, for example for maintenance purposes, then the aforementioned cooling equipment presents a critical technical problem for a superconducting magnet. During the replacement process, air or other gases may freeze solid on the very cold contact surface, in this case the holding structure of the superconducting windings. Ice formed at these positions leads to a poor thermal connection of the subsequently reused cold head to the holding structure of the winding.
In order to prevent gases from freezing solid on the very cold contact surfaces, these may be heated to about room temperature. The effect of this is generally that all the parts of a device which are to be cooled, for example the entire superconducting windings of a magnet, must be brought to room temperature before the cold head can be replaced. Particularly for large systems, such a heating phase and the subsequent cooling phase may take a long time. This leads to long down-times of the system. The heating and cooling phases furthermore lead to great consumption of energy.
As an alternative, the freezing of ambient gases on the very cold contact surfaces may be avoided by deliberately flooding the space around these contact surfaces with gas. This is elaborate, however, and leads to great consumption of flushing gas or refrigerant evaporated for this purpose.
EP 0 696 380 B1 discloses a superconducting magnet with a refrigerant-free refrigeration apparatus. The disclosed refrigeration apparatus has a heat bus made of a material with high thermal conductivity, for example copper, which is connected to the superconducting magnet. The heat bus can furthermore be connected to two cold heads. The two cold heads are arranged symmetrically with respect to the heat bus. They can respectively be brought onto the heat bus from opposite sides. In this way, one or both cold heads can be brought in thermal contact with the heat bus. The refrigerating power is correspondingly introduced from one or both cold heads into the heat bus.
In order to replace one of the two cold heads of the aforementioned apparatus, it may be mechanically retracted from the heat bus so that the corresponding cold head is likewise thermally separated from the heat bus. In this case, the refrigerating power is provided merely by the one remaining cold head. The retracted cold head may now be replaced without the superconducting magnet having to be heated. In the refrigeration apparatus disclosed in EP 0 696 380 B1, however, the cold heads must be rendered mechanically mobile, which requires a multiplicity of low-temperature compatible movable components and a corresponding, possibly error-prone, mechanism.
DE 102 11 568 B4 discloses a refrigeration apparatus having two cold heads which are connected via a pipeline system, in which a refrigerant can be circulated by a thermosiphon effect, to the parts of a device which are to be cooled. The pipeline system has a bifurcation. On each of the ends of the branches, there is a refrigerant space which is respectively connected to a cold head. Driven by gravity, liquid refrigerant flows down from one of these refrigerant spaces to the parts of the device which are to be cooled, where the thermal transfer takes place. Gaseous refrigerant rises back through the pipeline system to the two cold heads, where it is reliquefied. Such a cycle of the refrigerant can take place in the pipeline system both in the event that only one cold head is operating, and in the event that both cold heads are operating. If the refrigeration apparatus is dimensioned so that even a single cold head can deliver the refrigerating power needed for the parts of the device which are to be cooled, then the other (second) cold head may be replaced during continuous operation of the refrigeration apparatus. In order to minimize thermal losses, the pipeline system is made of a material with low thermal conductivity between the bifurcation and the refrigerant spaces, each of which is connected to a cold head. In this way, the losses due to solid-state thermal conduction in the branches between the bifurcation and the respective refrigerant space can be limited. Some gaseous refrigerant, however, will still also rise to the point where there is no cold head, or a cold head which is switched off. Thus, although the losses due to solid-state thermal conduction can be limited, the losses which are due to recirculating refrigerant cannot.